1. Technical Field
The present invention relates to a method of manufacturing a screen cylinder, and a screen cylinder that is particularly suitable for screening, filtering, fractionating, or sorting cellulose pulp or fiber suspensions of the pulp and paper industry, or other similar suspensions. The present invention relates more particularly to screening or filtering devices of the type comprising a plurality of screen wires positioned at a small spacing parallel to each other, the plurality of screen wires forming a screening or filtering surface facing the pulp or fiber suspension to be screened and adjacent wires forming screening openings therebetween allowing an accept portion of the pulp or fiber suspension to flow therethrough.
2. Description of Related Art
The first wire screens that appeared on the market had screen wires welded onto support rods or wound around the rods. One problem relating to such a structure is the positioning of the screen wires on the support rods such that the spacing i.e. the screening slots between the adjacent wires are substantially equal all over the screening surface. This problem was solved in the next generation wire screens by machining or otherwise arranging notches at desired intervals in the surface of the support rods the wires were supposed to be fastened. Now by attaching the screen wires to the notches the spacing between the wires is the desired one.
For instance, EP-A1-0 929 714 discusses a screening device in which the screen wires are fixed on the downstream side of the wires to transversely extending notches in solid support elements, i.e. support rings or support bars.
In known screening devices of this type, the support elements, which form the supports for the screen wires, are formed of solid bars, mainly rectangular but sometimes round or rounded in their cross section and most typically positioned perpendicular to the screen wires. However, the above-mentioned EP-A1-0 929 714 discloses a wire screen where the support ring is of specific construction, i.e. it is a U-shaped bar, to which the screen wires are attached by means of deformation in notches machined transverse to the support bar.
The screen wires are generally fastened to the support bars by a welding process which gives rise to a number of disadvantages such as variability distortion, thermal stresses and burrs. The heat induced by the welding often causes distortion of the wires and changes in the screening opening width between adjacent wires. It is therefore difficult to get completely uniform screening openings, which means that the efficiency of the screen suffers. Today, when the desired width of screening openings may be as small as 0.1 mm, or even smaller, only minimal distortions (if any) are acceptable.
The thermal stresses and the burrs may also lead to failure in operation due to the loading on the screening device in the users process. Such loading may be either in the form of a constant load or a cyclic loading giving rise to failure by fatigue. Burrs may also catch fibers of the suspension, leading to gradual clogging of the screen or filter, or the formation of so-called strings of fibers attached to each other which are very detrimental in the user's process.
Now that the use of notches in the support rods, bars or rings ensures that the distance between the adjacent screen wires is substantially constant, the next problem to solve is to find out how the screen wires could be fastened to the support rods or bars such that the fastening by means of welding would not bring any additional problems. The welding has been a reliable and simple way of securing the screen wires in the notches in the support rods or bars in such a manner that the wires would not be able to move in the notch. However, since the welding is apt to create some additional problems listed already above, a so called keyhole notch has been tested for replacing the welding. The keyhole notch or opening is machined either entirely inside the support element, or ring or bar, or machined such that the keyhole is open at one side of the support element or bar or ring. In both cases the wire is able to move in the notch only in the direction of its longitudinal axis. In other words, the keyhole either clamps the wire substantially tightly, or allows the wire to be slid into the keyhole in the direction of the longitudinal axis of the wire. Thus it is clear that the keyhole prevents the screen wire from moving in the direction of the pressure pulses created during the screening i.e. in the direction substantially perpendicular to the screen wires.
U.S. Pat. Nos. 5,090,721 and 5,094,360, for instance, suggest the attachment of screen wires by means of a certain keyhole cross section into notches in the support bar having the same keyhole form. The screen wires are inserted in the notches while the support bars are straight, i.e. not bent. By bending the support bars into support rings, the screen wires are clamped into the notches. This design, however, may not be reliable enough in the long run, and the keyhole fastening together with the clamping feature has been improved with a number of suggestions known better in the industry. In other words, gluing, soldering, welding etc. have been suggested to ensure the keyhole fastening.
The assembling of the screen wires to keyholes or notches in the support ring has been improved by using heat treatment of the support ring (see for instance U.S. Pat. No. 5,394,600). Both heating of the support bar before rounding such to a support ring or heating of the support ring has been suggested. The idea has been to heat the entire support element (bar or ring) to expand the notches such that the screen wires may be inserted into the notches. In other words, the heating has been performed prior to the screen wire installation, and the entire support element has been heated. However, such a heat treatment has not proven to be such a reliable way of fastening of the screen wires to the support rings that the fastening could manage without any further ways (listed already above) of ensuring the fastening.